my camera is a 15 mp camera, therefore it takes pictures up to 4752x3168 pixels. When I open a picture in photoshop and look into the image size it will say 72 dpi and 66 inches width by 44 inches height. I am used to changing the resolution to 300 and so it will automatically change the width and height to 15.84x10.56 inches but the pixels of course will remain the same(4752x3168). I also change it to CMYK. So my question is what if I want to print a large format (30 x20 inches) should I live the image size as it is when opened, in 72 dpi? or should always change the resolution to 300? or should I change it to 30x20 and link the resolution so that the pixels will remain the same but the resolution will automatically change accordingly. In this case 158.4. Is it ok to print photographs in CMYK? what kind of printing companies print large format pictures?

my camera is a 15 mp camera, therefore it takes pictures up to 4752x3168 pixels. When I open a picture in photoshop and look into the image size it will say 72 dpi and 66 inches width by 44 inches height. I am used to changing the resolution to 300 and so it will automatically change the width and height to 15.84x10.56 inches but the pixels of course will remain the same(4752x3168). I also change it to CMYK. So my question is what if I want to print a large format (30 x20 inches) should I live the image size as it is when opened, in 72 dpi? or should always change the resolution to 300? or should I change it to 30x20 and link the resolution so that the pixels will remain the same but the resolution will automatically change accordingly. In this case 158.4. Is it ok to print photographs in CMYK? what kind of printing companies print large format pictures?

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For digital photographs the term is ppi, not dpi. The ppi of an image is meaningless for computer or online display.
For printing the ppi and the pixel dimensions of the image determine the print size.

A 4752x3168 px image printed at 100 ppi will be 47.52" X 31.68".
A 4752X3168 px image printed at 300 ppi will be 15.84" X 10.56".

Many print lab printers only output images at a maximum of 250 ppi.

Here is a link to a popular print labs section on how to prepare images for printing: Mpix.com - Help

CMYK is used to print press products. Photo print labs use a chromogenic printing processed based on sRGB.